Liquid crystal displays (LCDs) are commonly used in laptop computers, desktop monitors, and television applications. One embodiment of the present invention deals with a color, transmissive LCD that requires backlighting, where the backlight contains red, green, and blue LEDs.
FIG. 1 is a cross-sectional view of a color, transmissive LCD 10 that includes a backlight 12. The backlight contains an array of red, green, and blue LEDs 14 whose combined light forms white light.
The backlight 12 ideally provides homogenous light to the back surface of the display. Providing homogenous white light using physically spaced LEDs is very difficult in a shallow backlight box. The backlight box has diffusively reflective bottom and side walls to mix the red, green, and blue light. The inner surfaces may be painted white. Mixing optics 16, such as a diffuser sheet, improves the color mixing. A diffuser sheet may simply be a roughened sheet of plastic. A brightness enhancement sheet may also be used to direct most of the light within a narrow field of view.
Above the mixing optics 16 are conventional LCD layers 18, typically consisting of polarizers, RGB filters, a liquid crystal layer, a thin film transistor array layer, and a ground plane layer. The electric fields created at each pixel location, by selectively energizing the thin film transistors at each pixel location, causes the liquid crystal layer to change the polarization of the white light at each pixel location. The RGB filters only allow the red, green, or blue component of the white light to be emitted at the corresponding RGB pixel locations. LCDs are well known and need not be further described.
As LED technology advances, the light output and efficiency of power LEDs increase, and fewer LEDs are needed to provide the required light output for an LCD. Using fewer LEDs typically reduces the cost of the backlight. Increasing the pitch of the different color LEDs makes it more difficult to provide adequate color uniformity across the LCD screen, especially with a relatively shallow backlight box. In a 40 inch television backlight, each LED may be spaced apart 10-25 mm.
LEDs in a backlight are typically arranged in rows, where a row is formed of repeating clusters of linearly-arranged LEDs (e.g., GRBG GRBG . . . ). In the prior art, each cluster is designed to have the same target white point, which is the resulting combined light from the cluster assuming total color mixing. A white point is typically identified by a correlated color temperature (CCT) in Kelvin. Other parameters for identifying a white point may also be used and are well known. In the 4-LED cluster of GRBG, the red LED is to the left of the center of the cluster, so the light to the left of center has a reddish tint (the center of gravity of the red is to the left of the center of the cluster). Similarly, since the blue LED is to the right of the center, the light to the right of center has a bluish tint (the center of gravity of the blue is to the right of the center of the cluster). However, for clusters arranged in horizontal rows, when a GRBG cluster is surrounded by other GRBG clusters, which occurs near the middle of the backlight, the spatial nonuniformity of the white point of a single cluster is offset by the opposite spatial nonuniformity of the white points of adjacent clusters. Thus, near the middle of the backlight, there is good color uniformity after some color mixing by the cavity and diffusion by the top diffuser.
Applicants have determined that the offsetting of the spatial nonuniformity of a cluster does not occur for the clusters at the left and right edges of the backlight. Accordingly, the left and right edges of the backlight have the worst color uniformity, which is perceptible to the human eye. Prior art FIGS. 3A and 3B, discussed further below, illustrate an example of this color uniformity problem.
Although a thicker backlight box may provide the additional color mixing to ameliorate these edge effects, thick backlights are undesirable.
Therefore, new techniques are needed for improving the color uniformity across a backlight using LEDs.